High molecular weight organo-functional endblocked polysiloxanes may be produced by condensation polymerisation of relatively low molecular weight linear silanol functional organosiloxanes in the presence of a low molecular weight organo-functional endblocked organosiloxane and a suitable catalyst. For example, GB Patent 2,252,975 describes a method for making organopolysiloxanes from organosiloxanes using a phosphonitrile halide catalyst. A problem associated with such condensation polymerisation processes is the amount of residual silanol functional polyorganosiloxane remaining in the reaction mixture after completion of the polymerisation reaction, which amount tends to increase as a function of the viscosity of the desired product polyorganosiloxane. There has thus been a need for an effective silylating agent which can react with such residual silanol to afford the desired functionality, and which can also react with other silanol groups, such as those on the surface of silica particles and other materials for use as filler in, for example, silicone rubber, and silanol on the surface of substrates such as glass. Silylacetamide is an effective silylating agent but generates acetamide, a solid by-product which is difficult to remove from the reaction mixture. Cyclic trisilazane is another effective silylating agent but is also a chain extender which leads to broadening of molecular weight distribution. U.S. Pat. No. 4,059,559 discloses the use of disilazanes as silylating agents.